1. Field of the Invention
The invention relates to a method for temperature compensation of an LC oscillator having a machining head and serving for the capacitive measurement of a distance between the machining head and a workpiece, in which an output signal of the LC oscillator is transmitted to an evaluation unit arranged separately from the machining head. Further, the invention relates to a machining head and also to an associated evaluation unit for carrying out the method.
2. Description of the Related Art
It is already generally known for the distance between a workpiece and a machining head for machining the workpiece to be measured capacitively, to be precise using a sensor electrode which is electrically conductively connected to the machining head and yields a sensor signal which corresponds to the distance and whose magnitude depends on the capacitance between sensor electrode and workpiece.
By monitoring the sensor signal, it is possible to position the machining head relative to the workpiece in order to be able to machine the workpiece in a suitable manner. In this case, the positioning is effected by means of a regulating device which receives the measured distance as actual value and controls the position of the sensor electrode or the machining head in a manner dependent on a comparison of the actual value with a predetermined desired value.
The machining head may be a laser cutting nozzle, for example, from which a laser beam emerges for machining the workpiece. With the aid of the laser beam, welding work, cutting work, etc, can be carried out on the workpiece.
However, previously known systems in which an LC oscillator is used for capacitive distance measurement, and one of which is disclosed e.g. in German patent application 198 47 365.6, have a temperature drift originating from the temperature coefficient of the coils used. This temperature drift has the effect of corrupting the measured distance between machining head and workpiece and is essentially caused by the fact that the components of the LC oscillator are heated by the welding or cutting process taking place not very far away.
In order to be able at least partially to eliminate this temperature drift, it has already been conceived of to insert additional capacitors into the resonant circuit of the LC oscillator in order to be able to compensate the temperature drift of the coils by the temperature coefficient of the capacitors. However, this has the disadvantage that the additional capacitors are in parallel with the measuring capacitor formed from sensor electrode and workpiece, and thus reduce the measurement sensitivity. A further disadvantage is that the variation of the temperature coefficients of individual components cannot be taken into account and the matching of the oscillator circuit during development is very time-consuming.
The invention is based on the object of providing a further method for the temperature compensation described, which can be performed more simply and more accurately. Furthermore, the intention is to provide a machining head which is suitable for carrying out the method, and also an evaluation unit suitable therefor, to which the processing head can be connected.
A method according to the invention for temperature compensation of an LC oscillator having a machining head is distinguished by the fact that the temperature of the LC oscillator is measured with the aid of a temperature sensor, that the temperature is transmitted to the evaluation unit, and that, in the evaluation unit, temperature compensation of the output signal of the LC oscillator is effected using the measured temperature and a temperature coefficient assigned to the machining head.
According to the invention, then, the temperature drift of the resonant circuit of the LC oscillator is no longer compensated at the resonant circuit itself, rather the temperature is transmitted to the evaluation unit so that the temperature compensation can be performed there. Accordingly, there is also no longer a need to insert into the LC generator additional components whose temperature coefficient itself cannot be taken into account and which would only lead to a reduction in the measurement sensitivity of the system. The method according to the invention thus leads to more accurate results and can be carried out more simply.
The temperature coefficient which is required for temperature compensation and is assigned to a respective machining head can be recorded empirically after the production of the machining head and be stored at a suitable location. If the evaluation unit knows what machining head is involved, then the temperature coefficient assigned to the machining head can be stored e.g. in the evaluation unit and be retrieved by the latter as required. However, for the sake of simpler handling, after the production of the respective machining head, the associated temperature coefficient of the LC oscillator can also be stored in a storage device which is connected to the machining head itself. Consequently, the assignment between machining head and temperature coefficient is always unambiguous and can lead to more reliable temperature compensation. If the temperature coefficient is required for compensation purposes, then it can be transmitted to the evaluation unit in a suitable way, to be precise together with the temperature measured by the temperature sensor, or the corresponding measured temperature value. The temperature coefficient may be stored e.g. in a non-volatile memory. The transmission of the measured temperature value to the evaluation unit and, if appropriate, the transmission of the temperature coefficient to the evaluation unit may be carried out periodically or at predetermined time intervals.
According to a highly advantageous refinement of the invention, the transmission of the measured temperature (the measured temperature value) and/or of the stored temperature coefficient to the evaluation unit is effected digitally, since this enables the transmission to be carried out very accurately. For this purpose, the data format of the digital transmission may be a serial bit stream containing, in coded form, the measured temperature value and, if appropriate, the temperature coefficient. The customary checking methods for ascertaining entirely satisfactory data transmission from the machining head to the evaluation unit can then be applied to it, if necessary. However, for the digital transmission of measured temperature value and, if appropriate, the temperature coefficient, a square-wave voltage may also be used whose frequency is proportional to the measured temperature and whose pulse duration is proportional to the temperature coefficients. In such a case, the transmission device can be constructed relatively simply, for instance using a monostable multivibrator.
According to a further, highly advantageous refinement of the invention, at least the output signal of the LC oscillator and the measured temperature, but also, if appropriate, the temperature coefficient, are transmitted from the machining head to the evaluation unit via the same line, which results in a particularly simple connection between machining head and evaluation unit. For this connection, it is suitable to use a coaxial cable. In this case, the transmission of the measured temperature and/or of the temperature coefficient can be effected digitally in a different frequency range from that of the output signal of the LC oscillator, in order to avoid interference between the two signal groups.
In a development of the invention, the digitization may be effected by modulation of the current demand of a circuit which is present in a manner pertaining to the machining head and contains the temperature sensor, the modulated current demand being monitored in the evaluation unit. From the modulated current demand, the evaluation unit can thus draw conclusions about the measured temperature and/or about the temperature coefficient in order then to use these values as a basis for further machining.
A machining head having an LC oscillator formed partly by it and serving for the capacitive measurement of a distance between the machining head and a workpiece to be machined, in which an output signal of the LC oscillator can be transmitted to an evaluation unit separate from the machining head, is distinguished by the fact that it has at least one temperature sensor for measuring the temperature of the LC oscillator, and that the measured temperature can be transmitted to the evaluation unit. In this case, it may also have a storage device for storing a temperature coefficient which is provided for it and can likewise be transmitted to the evaluation unit.
For digitization and thus reliable transmission of the measured temperature and/or of the temperature coefficient, use is made of a digitization circuit which is present on the machining head and can be supplied via a cable with a supply voltage from the evaluation unit, the digitization, for the sake of simplicity, being effected by modulation of the current demand of the digitization circuit. In this case, the modulation of the current demand of the digitization circuit can be carried out by connecting a resistor to the supply voltage. The evaluation unit is then designed in such a way that it has a current measuring device for measuring the supply current of the machining head, and a comparison circuit connected downstream of the current measuring device. By comparing the measured current values of the supply current with a reference current value provided by the comparison circuit, it is thus possible to reproduce the binary data for the transmitting measured temperature value and/or the transmitting temperature coefficient in the evaluation unit, and from this then the corresponding actual values in order then to correct the measured distance signal (frequency signal) in correspondence with the measured temperature value and the temperature coefficient. This can be done in a microprocessor of the evaluation unit, which on the one hand is connected to the output of the comparison circuit and on the other hand receives the output signal of the LC oscillator via a high-pass filter.
It shall also be pointed out that all data are preferably transmitted via the inner conductor of a coaxial cable from the machining head to the evaluation unit, and that the machining head can be designed in any desired way. For instance, it may be a laser machining head, through which, therefore, a laser beam passes which is used for carrying out welding or cutting work, or, alternatively, it may be in a machining head in which, for instance, only a machining gas emerges.